1. Field of the Invention
The present invention relates to variable resistors and in particular to a ceramic resistor card for use in a transducer for converting a physical position into an electrical signal.
2. Description of the Related Art
Variable resistors are known for sensing parameters in a variety of applications. For example, the fuel level in an automobile tank is typically measured using a variable resistor having a sweep arm mechanically or electrically coupled to a float located in a fuel tank. The sweep arm position is determined by the level of fuel in the tank and the voltage value sensed across the variable resistor depends on the position of the sweep arm.
The variable resistor typically comprises a resistor card with metalized areas and thick film resistive ink. The thick film ink is deposited in precise areas to interconnect respective metalized areas. There is a wiper blade assembly having contact fingers thereon connected to the sweep arm. As the float raises and lowers according to the fluid level, the wiper contact fingers move along the resistor card in an arcuate path and make corresponding contact with the metalized areas. This results in a voltage change that generates a signal representative of the amount of fuel contained in the tank.
One problem with this type of system is that over the life of a vehicle it must go through thousands of cycles in a harsh environment. The card is often exposed to both fuel and dry conditions as the fluid level is decreased. In addition, the assembly is exposed to severe vibration resulting from the vehicle movement. The result is that wear occurs as the wiper contact fingers go back and forth over the metalizations and the electrical resistance of the metalization portions may increase and cause accuracy problems with the fuel reading. Additionally, the metalized portions may wear to the point that an open circuit results along some given or several metalization locations.
Mother problem in some systems is that small circuit breaks occur as the arm sweeps from one metalization portion to the next. These breaks can cause spikes in the electrical characteristics of the variable resistor. A filter may have to be placed in the circuitry to minimize the spiking problem. It is desirable to eliminate the spiking and therefore the need for any associated filters.